Optical modulator

ABSTRACT

The invention provides an optical modulator which suppresses the loss of a microwave which advances through an electrode and of light which propagates in a waveguide and makes the losses in individual arm waveguides substantially equal to each other to suppress the deterioration of the extinction coefficient to improve the transmission quality. The optical modulator includes a substrate having an electro-optical effect and having a ridge, first and second grooves and first and second banks formed thereon, a Mach-Zehnder optical waveguide, an electrode, and first and second recesses formed at symmetrical positions with respect to the ridge on the first and second banks, respectively. The optical modulator is applied, for example, to a transmission side apparatus for a long distance optical transmission system.

This application is a divisional of application Ser. No. 09/820,634,filed Mar. 30, 2001, now U.S. Pat. No. 6,584,240.

BACKGROUND OF THE INVENTION

1) Field of the Invention

This invention relates to an optical modulator suitable for use in thefield of long distance optical communication systems.

2) Description of the Related Art

As the data transmission rate increases in recent years, opticalmodulators for modulating a data signal from an electric signal into anoptical signal are developed energetically in the field of long distancecommunication systems for communication such as submarine opticalcommunication.

One of such optical modulators as just described is, for example, such asingle drive optical modulator 20 as shown in FIG. 8. Referring to FIG.8, the single drive optical modulator 20 shown includes a substrate 21on which a Mach-Zehnder optical waveguide 22 is formed, and an electrode23 formed integrally on the substrate 21 and including a single signalelectrode 23A and a grounding electrode 23B.

FIG. 9 is a sectional view taken along line A-A′ of the single electrodeoptical modulator 20 shown in FIG. 8. As seen in FIG. 9, the singleelectrode optical modulator 20 is configured such that the electrode 23is formed on the substrate 21, which is made of, for example, lithiumniobate (LiNbO₃) and cut (Z-axis cut) in the Z-axis direction of thecrystal orientation, together with the Mach-Zehnder optical waveguide22.

The Mach-Zehnder optical waveguide 22 is formed by thermal diffusion oftitanium (Ti) or a like substance on the substrate 21 and includes a Ybranching waveguide 22A and two straight arm waveguides 22B-1 and 22B-2on the light incoming side and a Y branching waveguide 22C on the lightoutgoing side. The electrode 23 includes the single signal electrode 23Aand the grounding electrode 23B and converts, when a voltage signal(microwave) of, for example, NRZ data or the like is applied to thesignal electrode 23A, the voltage signal into an NRZ optical signal.

As shown in FIG. 8, the single signal electrode 23A is formed so as toestablish electric connection between two connector contacts on aone-side edge portion of the substrate 21 in its widthwise direction,and is formed such that part of it extends along and above the portionat which the straight arm waveguide 22B-1 is formed. Further, thegrounding electrode 23B is formed such that it is disposed on theopposite sides of the single signal electrode in a spaced relationshipby a predetermined distance thereby to form a coplanar line on thesubstrate 21.

When light from a light source not shown is introduced into the singleelectrode optical modulator 20 having the configuration described abovewith reference to FIGS. 8 and 9, while the light propagates in theMach-Zehnder optical waveguide 22, it is modulated into an NRZ opticalsignal by the signal electrode 23A to which a voltage signal (microwave)of NRZ data or the like is applied. The modulated NRZ optical signalgoes out of the single electrode optical modulator 20.

Where such a single electrode optical modulator 20 as described above isused to modulate a voltage signal into a data optical signal of atransmission rate particularly of 10 Gb/s or more, preferably ofapproximately 40 Gb/s, it is a significant subject for improvement ofthe transmission quality to suppress the loss of a microwave whichadvances through the electrode and suppress the deterioration of theextinction ratio.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an optical modulatorwhich suppresses the loss of a microwave which advances through anelectrode and loss of light which propagates in a waveguide and makesthe losses in individual arm waveguides substantially equal to eachother to suppress the deterioration of the extinction ratio to improvethe transmission quality.

In order to attain the object described above, according to an aspect ofthe present invention, there is provided an optical modulator,comprising a substrate having an electro-optical effect and havingformed thereon a ridge, first and second grooves which are positioned onthe opposite sides of the ridge, and first and second banks which arepositioned on the outer sides of the first and second grooves,respectively, a Mach-Zehnder optical waveguide formed on the substrateand including a first Y branching waveguide, first and second armwaveguides which are branched from the first Y branching waveguide andone of which is included in the ridge, and a second Y branchingwaveguide at which the first and second arm waveguides join together, anelectrode formed on the substrate and including a signal electrodeformed on the ridge and a grounding electrode formed on the first andsecond banks for controlling light propagating in the optical waveguide,and first and second recesses formed at symmetrical positions withrespect to the ridge on the first and second banks, respectively.

With the optical modulator, since the first and second recesses areformed at the symmetrical positions with respect to the ridge 14 a onthe first and second banks, respectively, also the electric fielddistribution in the substrate by an electric signal provided to thesignal electrode can be made symmetrical with respect to the ridge.Consequently, the optical modulator is advantageous in that the loss ofa microwave which advances through the signal electrode can besuppressed.

Preferably, the substrate is made of LiNbO₃, and more preferably, thesubstrate made of LiNbO₃ is a Z-axis cut substrate.

The optical modulator may be configured such that the groundingelectrode is provided on the first and second recesses and an air gap isformed in each of the first and second recesses or part of the groundingelectrode is filled in the first and second recesses.

Preferably, the ridge and the first and second banks have top faceswhich are set in a substantially same level with one another, and morepreferably, the first and second recesses have a depth set substantiallyequal to the depth of the first and second grooves.

Preferably, the signal electrode contacts with the ridge with a widthsmaller than the width of the ridge.

Preferably, a buffer layer is formed between the substrate and theelectrode, and more preferably, the buffer layer is provided also in thefirst and second recesses.

Preferably, a silicon layer is placed on the substrate, and morepreferably, the buffer layer is provided also in the first and secondrecesses.

The optical modulator is advantageous in that the absorption loss oflight which propagates in the optical waveguide can be suppressed by thebuffer layer and electric charge generated by a pyroelectric effect canbe made uniform by the silicon layer to suppress the variation of theoperating point by a temperature variation.

Further, since the buffer layer or the silicon layer is formed also inthe first and second recesses 13-1, 13-2, the optical modulator isadvantageous also in that adjustment of the characteristic impedance,which should be kept to a fixed value set in advance, and the speedmatching between a microwave and light can be performed readily bysetting of the thickness of the buffer layer or the silicon layer.

Preferably, one of the first and second arm waveguides is provided at alocation of the other one of the first and second banks nearer to theridge than a corresponding one of the first and second recesses.

With the optical modulator, since the one arm waveguide is providednearer to the ridge than the other recess, the structure of thesubstrate portion at which the arm waveguide which is not included inthe ridge is formed can be formed substantially same as the structure ofthe ridge. Therefore, the optical modulator is advantageous in that thelosses of the arm waveguides can be made substantially equal to eachother to suppress the deterioration of the extinction ratio.

According to another aspect of the present invention, there is providedan optical modulator, comprising a Z-axis cut substrate made of LiNbO₃and having formed thereon a ridge, first and second grooves which arepositioned on the opposite sides of the ridge, and first and secondbanks which are positioned on the outer sides of the first and secondgrooves, respectively, a Mach-Zehnder optical waveguide formed on thesubstrate and including a first Y branching waveguide, first and secondarm waveguides which are branched from the first Y branching waveguideand one of which is included in the ridge, and a second Y branchingwaveguide at which the first and second arm waveguides join together, anelectrode formed on the substrate and including a signal electrodeformed on the ridge and a grounding electrode formed on the first andsecond banks for controlling light propagating in the optical waveguide,a buffer layer formed between the substrate and the electrode, a siliconlayer placed on the substrate, and first and second recesses formed atsymmetrical positions with respect to the ridge on the first and secondbanks, respectively.

With the optical modulator, since the first and second recesses areformed at the symmetrical positions with respect to the ridge 14 a onthe first and second banks, respectively, also the electric fielddistribution in the substrate by an electric signal provided to thesignal electrode can be made symmetrical with respect to the ridge.Consequently, the optical modulator is advantageous in that the loss ofa microwave which advances through the signal electrode can besuppressed.

Preferably, the buffer layer or the silicon layer is provided also inthe first and second recesses. In this instance, the optical modulatoris advantageous also in that adjustment of the characteristic impedance,which should be kept to a fixed value set in advance, and the velocitymatch between a microwave and light can be performed readily by settingof the thickness of the buffer layer or the silicon layer.

Preferably, one of the first and second arm waveguides is provided at alocation of the other one of the first and second banks nearer to theridge than a corresponding one of the first and second recesses. In thisinstance, the structure of the substrate portion at which the armwaveguide which is not included in the ridge is formed can be formedsubstantially same as the structure of the ridge. Therefore, the opticalmodulator is advantageous in that the losses of the arm waveguides canbe made substantially equal to each other to suppress the deteriorationof the extinction ratio.

The above and other objects, features and advantages of the presentinvention will become apparent from the following description and theappended claims, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a cross sectional structure of anoptical modulator according to several embodiments of the presentinvention;

FIG. 2 is a schematic view showing an optical modulator according to afirst embodiment of the present invention;

FIG. 3 is a schematic view showing an optical modulator according to asecond embodiment of the present invention;

FIG. 4 is a schematic view showing an optical modulator according to athird embodiment of the present invention;

FIG. 5 is a schematic view showing an optical modulator according to afourth embodiment of the present invention;

FIGS. 6 and 7 are schematic views showing optical modulators accordingto different modifications to the embodiments;

FIG. 8 is a schematic view showing a single drive optical modulator; and

FIG. 9 is a sectional view taken along line A-A′ of the opticalmodulator shown in FIG. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the present invention will be explained referring tothe accompanying drawings.

a. First Embodiment

FIG. 2 is a schematic view showing an optical RZ modulator 1 of theclock modulation type to which an optical modulator 12-1 according to afirst embodiment of the present invention is applied. Referring to FIG.2, the optical modulator 1 shown is used, for example, with atransmission side apparatus of a long distance optical transmissionsystem and modulates light from a light source (semiconductor laser) notshown with a transmission data signal. The modulated optical signal Istransmitted to the reception side through an optical fiber or the likenot shown.

The optical modulator 1 includes a Mach-Zehnder optical modulator 11-1and another Mach-Zehnder optical modulator 12-1 according to the firstembodiment integrated together with each other. The optical modulator11-1 can modulate light from the light source not shown into an opticalRZ clock of, for example, approximately 40 GHz, and the opticalmodulator 12-1 can modulate the optical RZ clock from the opticalmodulator 11-1 into an optical RZ data signal of, for example, 40 Gb/s.

FIG. 1 is a sectional view of the optical modulator 12-1 taken alongline R-S of FIG. 2. The optical modulator 12-1 shown has characteristicelements of the present invention denoted by reference characters 13-1and 13-2 in FIGS. 1 and 2. However, an overall structure of the opticalmodulator 1 is described first, and then the characteristic elements ofthe present invention are described.

Referring to FIG. 2, the optical modulator 1 includes a Mach-Zehnderfirst optical waveguide (hereinafter referred to simply as first opticalwaveguide) 5, a Mach-Zehnder second optical waveguide (hereinafterreferred to simply as second optical waveguide) 6, a first electrode7A-1 and a second electrode 7B-1 formed on a substrate 1A, which is madeof lithium niobate (LiNbO₃) and cut in the Z-axis direction of thecrystal orientation (Z-axis cut), and integrated together in one chip.

It is to be noted that, while the optical waveguides 5 and 6 and theelements denoted by reference characters 13-1 and 13-2 hereinafterdescribed are covered, at portions thereof which overlap with agrounding electrode 7, with the grounding electrode 7, in order toclearly indicate the arrangement of them, also the portions of themcovered with the grounding electrode 7 are indicated by solid lines inFIG. 2.

The first optical waveguide 5 and the second optical waveguide 6 areformed integrally by thermal diffusion of titanium (Ti) or the like sothat the second optical waveguide 6 may be in a relationship of cascadeconnection or series connection to the first optical waveguide 5.Consequently, light from the light source is inputted from the input endof the optical modulator 1 and propagates along the first opticalwaveguide 5 and the second optical waveguide 6.

The first optical waveguide 5 includes a Y branching waveguide 5A andtwo straight arm waveguides 5B-1 and 5B-2 on the light incoming side,and a Y branching waveguide 5C on the light outgoing side. Similarly,the second optical waveguide 6 includes a Y branching waveguide 6A andtwo straight arm waveguides 6B-1 and 6B-2 on the light incoming side,and a Y branching waveguide 6C on the light outgoing side.

In particular, the first optical waveguide 5 is formed on the substrate1A such that the Y branching waveguide 5A is branched into the twostraight arm waveguides 5B-1 and 5B-2 and then the straight armwaveguides 5B-1 and 5B-2 join together at the Y branching waveguide 5C.The second optical waveguide 6 is formed on the substrate 1A such thatthe Y branching waveguide 6A is branched into the two straight armwaveguides 6B-1 and 6B-2 and then the straight arm waveguides 6B-1 and6B-2 join together at the Y branching waveguide 5C.

The first electrode 7A-1 is formed as a partial electrode layer on thesubstrate 1A to control light which propagates in the first opticalwaveguide 5, and the second electrode 7B-1 is formed as a partialelectrode layer on the substrate 1A to control light which propagates inthe second optical waveguide 6.

The first electrode 7A-1 includes a dual electrode having two signalelectrodes 7 a-1 and 7 a-2, and grounding electrodes 7. The secondelectrode 7B-1 includes a signal electrode 7 b and the groundingelectrodes 7.

The signal electrodes 7 a-1 and 7 a-2 of the first electrode 7A-1 areformed so as to establish electric connection between two connectorcontacts at edge portions on the opposite sides of the substrate 1A inits widthwise direction. The signal electrode 7 a-1 is formed such thatpart thereof extends along and above the portion at which the straightarm waveguide 5B-1 of the first optical waveguide 5 is formed, and thesignal electrode 7 a-2 is formed such that part thereof extends alongand above the portion at which the other straight arm waveguide 5B-2 ofthe first optical waveguide 5 is formed.

The signal electrode 7 b formed on the second optical waveguide 6 isformed so as to establish electric connection between two connectorcontacts at edge portions on one side of the substrate 1A in itswidthwise direction. The signal electrode 7 b is formed further suchthat part thereof extends along and above the portion at which thestraight arm waveguide 6B-1 of the second optical waveguide 6 is formed.

The grounding electrode 7 is formed as common grounding electrodes forthe first electrode 7A-1 and the second electrode 7B-1 such that it ispositioned on the opposite sides the signal electrodes 7 a-1, 7 a-2 and7 b and bias electrodes bias electrode 7C-1, bias electrode 7C-2 and 7D,which are hereinafter described, in a spaced relationship by apredetermined distance to form a coplanar line on the substrate 1A.

A connection pad 7 d is formed with a comparatively great width for anelectric wiring line and serves as a connector contact for each of thesignal electrodes 7 a-1, 7 a-2 and 7 b described above. The biaselectrodes bias electrode 7C-1 and bias electrode 7C-2 serve as a dualbias electrode connected to a dc power supply 7F to supply a biasvoltage to the first optical waveguide 5 to supplementarily provide anapplication voltage for clock modulation.

The bias electrode bias electrode 7C-1 is formed such that it extendsalong and above a branch portion of the Y branching waveguide 5C on thestraight arm waveguide 5B-1 side, and the bias electrode bias electrode7C-2 is formed such that it extends along and above a branch portion ofthe Y branching waveguide 5C on the straight arm waveguide 5B-2 side.The bias electrode 7D applies a dc voltage from a dc power supply 7E asa single electrode to the second optical waveguide 6 to auxiliaryprovide an application voltage for NRZ optical modulation and is formedsuch that it extends along and above the straight arm waveguide 6B-1.

It is to be noted that, also when the modulation characteristic isvaried by a temperature variation or the like, the modulation efficiencycan be kept optimally by the bias voltages from the bias electrodes biaselectrode 7C-1, bias electrode 7C-2 and 7D described above.

Also the bias electrodes bias electrode 7C-1, bias electrode 7C-2 and 7Dhave a connection pad 7 d similarly to the signal electrodes 7 a-1, 7a-2 and 7 b described above.

A cutaway portion 1B is formed at a connection path 1C between theoptical modulator 11-1 and the optical modulator 12-1 such that it cutsaway the grounding electrode 7 so that the loss of light whichpropagates along the connection path 1C may be reduced.

In the optical modulator 1 according to the first embodiment, a bufferlayer (refer to reference character 1D of FIG. 2 which is hereinafterdescribed) is interposed between a face of the substrate 1A and theelectrode layers, and a silicon (Si) layer (refer to reference character1E of FIG. 2) is placed on the substrate 1A, more particularly on thebuffer layer 1D.

A clock generation drive section 8A generates a sine wave signal of afrequency of, for example, 20 GHz. The sine wave signal of 20 GHzgenerated is applied through the connection pad 7 d to the signalelectrode 7 a-1 of the dual electrode.

A phase delaying section 9A delays a clock signal from the clockgeneration drive section 8A by a time (τ) corresponding to apredetermined phase (180 degrees) and converts the voltage of the clocksignal. The clock signal of 20 GHz from the phase delaying section 9A isapplied through the connection pad 7 d to the other signal electrode 7a-2 of the dual electrode.

Consequently, the optical modulator 11-1 composed of the first opticalwaveguide 5 and the first electrode 7A-1 modulates light from the lightsource so that an optical clock signal of 40 GHz is propagated on theoutput side (refer to reference character C′ of FIG. 1) of the Ybranching waveguide 5C.

In particular, the clock generation drive section 8A and the phasedelaying section 9A described above produce, when a clock signal of afrequency (20 GHz) equal to one half the transmission rate (40 Gb/s) perunit time of output light of the optical RZ modulator 1 of the clockmodulation type is applied to the first electrode 7A-1, an optical RZsignal of a rate equal to the transmission rate (40 Gb/s) per unit timeof the output light of the optical RZ modulator 1 of the clockmodulation type.

In other words, light which propagates in the straight arm waveguides5B-1 and 5B-2 of the first optical waveguide 5 is acted upon by anelectro-optical effect of an electric signal as a clock signal appliedto the first electrode 7A-1 so that an optical RZ clock of 40 GHz can beproduced on the output side 5C′ of the Y branching waveguide 5C.

An NRZ data signal generator 10 is connected to the second electrode7B-1 and supplies an NRZ data signal of, for example, approximately 40Gb/s to the second electrode 7B-1.

In particular, the optical modulator 12-1 composed of the second opticalwaveguide 6 and the second electrode 7B-1 can modulate an optical RZclock of 40 GHz from the first optical waveguide 5 with an NRZ datasignal of 40 Gb/s at a timing synchronized with the optical RZ clock sothat it can output an optical RZ data signal of 40 Gb/s.

More particularly, light which propagates in the straight arm waveguide6B-1 of the second optical waveguide 6 is acted upon by anelectro-optical effect of an electric signal applied to the secondelectrode 7B-1 so that an optical RZ data signal of 40 Gb/s is outputtedon the output side (refer to reference character 6C′ of FIG. 1) of the Ybranching waveguide 6C.

FIG. 1 is a sectional view taken along line R-S of the optical modulator12-1 shown in FIG. 2. Referring to FIG. 1, the substrate 1A has anelectro-optical effect as described hereinabove, and is made of lithiumniobate (LiNbO₃) and cut in the Z-axis direction of the crystalorientation (Z-axis cut).

The substrate 1A has formed thereon a ridge 14 a, a first groove 14 b-1and a second groove 14 b-2 which are positioned on the opposite sides ofthe ridge 14 a, and a first bank 14 c-1 and 14-2 which are positioned onthe outer sides of the first groove 14 b-1 and the second groove 14 b-2,respectively.

While the first arm waveguide 6B-1 of the second optical waveguide 6 isincluded in the ridge 14 a as hereinafter described, the second armwaveguide 6B-2 of the second optical waveguide 6 is provided at alocation of the second bank 14 c-2 nearer to the ridge 14 a than therecess 13-2 which is hereinafter described.

It is to be noted that, as described hereinabove, the second electrode7B-1 is formed on the substrate 1A for controlling light propagating inthe second optical waveguide 6 and includes the signal electrode 7 b andthe grounding electrodes 7. The signal electrode 7 b is formed on theridge 14 a such that the contact width We thereof with the ridge 14 a issmaller than the width Rw of the ridge 14 a. The grounding electrodes 7of the second electrode 7B-1 is formed on the first bank 14 c-1 and thesecond bank 14 c-2.

Further, the signal electrode 7 b is formed such that it has a heighth₁, but the width W₁₀ of the top face thereof is greater than thecontact width We thereof with the ridge 14 a. The grounding electrodes 7formed on the first bank 14 c-1 and the second bank 14 c-2 is formedwith an L-shaped sectional shape taken along line R-S in FIG. 2 suchthat it has a height h₂ at a portion thereof adjacent an end portion ofthe substrate 1A but has another height hw at a formation face thereofadjacent the ridge 14 a. It is to be noted that the signal electrode 7 bis formed such that the height of the portion thereof formed with thecontact width We is equal to the height hw of the formation face of thegrounding electrodes 7 adjacent the ridge 14 a.

Here, the ridge 14 a is formed with a height Gh such that it extends inparallel to the first arm waveguide 6B-1 including the portion at whichthe first arm waveguide 6B-1 on which the signal electrode 7 b is placedis formed. The first groove 14 b-1 and the second groove 14 b-2 areformed by partly removing or digging the substrate 1A to the depth Gh byetching.

The first bank 14 c-1 is formed by digging the first groove 14 b-1 onthe outer side of the first groove 14 b-1, i.e., on the outer side inthe widthwise direction of the substrate 1A, and the second bank 14 c-2is formed by digging the second groove 14 b-2 on the outer side of thesecond groove 14 b-2. i.e., on the outer side in the widthwise directionof the substrate 1A. The top face level of the ridge 14 a and the topface levels of the banks 14 c-1 and 14 c-2 are set substantially inlevel with each other.

The first bank 14 c-1 and the second bank 14 c-2 include a first recess13-1 and a second recess 13-2 as characteristic elements of the presentinvention, respectively, for reducing the loss in the waveguide andsuppressing the loss of a microwave which advances in the signalelectrode 7 b.

The first recess 13-1 and the second recess 13-2 are set to a depth Ghsubstantially equal to that of the first groove 14 b-1 and the secondgroove 14 b-2 and are formed at symmetrical positions with respect tothe ridge 14 a in the first bank 14 c-1 and the second bank 14 c-2,respectively, using etching.

More particularly, the first recess 13-1 and the second recess 13-2 canbe formed in a shape of a continuous groove of the width Gw2 such thatthey may extend in parallel to the first arm waveguide 6B-1 and thesecond arm waveguide 6B-2 and have a length substantially equal to thelength L of the grounding electrodes 7 on the upper face of the secondarm waveguide 6B-2 as indicated by broken lines in FIG. 2. In otherwords, the two recesses 13-1 and 13-2 described above can be formed suchthat they have a symmetrical relationship with respect to the line ofthe first arm waveguide 6b-1 which has the signal electrode 7 b formedon the upper face thereof.

The grounding electrodes 7 is formed also on the recesses 13-1 and 13-2,and an air gap is formed in each of the recesses 13-1 and 13-2, that is,a region defined by each of the recesses 13-1 and 13-2 and the groundingelectrodes 7.

The buffer layer 1D is provided for suppressing the absorption loss oflight which propagates in the optical waveguides 5 and 6 and is formedbetween the electrodes 7A-1 and 7B-1, which form the optical modulators11-1 and 12-1, respectively, and the substrate 1A.

A silicon layer 1E is formed on the substrate 1A, particularly on thebuffer layer 1D. The silicon layer 1E is effective to uniformizeelectric charge generated by a pyroelectric effect to suppress avariation of the operating point by a temperature variation.

The buffer layer 1D and the silicon layer 1E are provided also in therecesses 13-1 and 13-2 and facilitate adjustment of the characteristicimpedance, which should be kept to a fixed value set in advance, and thevelocity match between a microwave and light by setting of the thicknessof the buffer layer 1D and the silicon layer 1E.

When light from the light source not shown is introduced into theoptical modulator 1 of the first embodiment having the configurationdescribed above, while the light propagates in the first opticalwaveguide 5 which forms the optical modulator 11-1, it can be convertedinto an optical RZ signal of 40 GHz with a sine wave of 20 GHz by thefirst electrode 7A-1 to which an RZ signal of a frequency of 20 GHzgenerated by the clock generation drive section 8A is applied.

Further, while the optical RZ signal propagates in the second opticalwaveguide 6 which forms the optical modulator 12-1, an NRZ signal of 40Gb/s generated by the NRZ data signal generator 10 is applied to thesecond electrode 7B-1 to modulate the optical clock thereby to modulatethe optical RZ signal into an optical RZ data signal of 40 Gb/s. Theoptical RZ data signal is transmitted to the reception side through anoptical fiber or the like not shown.

Since the second recess 13-2 is provided on the second bank 14 c-2 onwhich the straight arm waveguide 6B-2 of the second optical waveguide 6described above is formed, the structure of the portion at which thesecond arm waveguide 6B-2 is formed can be formed substantially same asthe structure of the ridge 14 a which has the signal electrode 7 bprovided on the top portion thereof. Consequently, the losses by the armwaveguides 6B-1 and 6B-2 are substantially equal to each other.

Further, since the first recess 13-1 is provided at the positionsymmetrical with the second recess 13-2 with respect to the ridge 14 aon the first bank 14 c-1 on which the two arm waveguides 6B-1 and 6B-2are not formed as shown in FIG. 1, also the electric field distributionin the substrate 1A by an electric signal provided to the signalelectrode 7 b can be made symmetrical with respect to the ridge 14 a.

In this manner, with the optical modulator 12-1 according to the firstembodiment of the present invention, since the first recess 13-1 and thesecond recess 13-2 are formed at symmetrical positions with respect tothe ridge 14 a on the first bank 14 c-1 and the second bank 14 c-2,respectively, also the electric field distribution in the substrate 1Aby an electric signal provided to the signal electrode 7 b can be madesymmetrical with respect to the ridge 14 a. Consequently, the opticalmodulator 12-1 is advantageous in that the loss of a microwave whichadvances in the signal electrode 7 b can be suppressed.

Further, since the provision of the second recess 13-2 makes it possibleto form the structure of the portion at which the second arm waveguide6B-2 is formed substantially same as the structure of the ridge 14 awhich has the signal electrode 7 b formed at the top portion thereof,the optical modulator 12-1 is advantageous also in that the losses bythe arm waveguides 6B-1 and 6B-2 can be made substantially equal to eachother thereby to suppress the deterioration of the extinction ratio.

Furthermore, since the buffer layer 1D is formed between the face of thesubstrate 1A and the first electrode 7A-1 and second electrode 7B-1, theoptical modulator 12-1 is further advantageous in that the absolutionlosses of light which propagates in the optical waveguides 5 and 6 canbe suppressed and electric charge generated by a pyroelectric effect canbe made uniform by the silicon layer 1E formed on the substrate 1Athereby to suppress the variation of the operating point by atemperature variation.

In addition, since the buffer layer 1D and the silicon layer 1E areformed also in the recesses 13-1 and 13-2, the optical modulator 12-1 isadvantageous in that adjustment of the characteristic impedance, whichshould be kept to a fixed value set in advance, and the velocity matchbetween a microwave and light can be performed readily by setting of thethickness of the buffer layer 1D.

b. Second Embodiment

FIG. 3 is a schematic view showing an optical RZ modulator of the clockmodulation type to which optical modulators 11-2 and 12-1 according to asecond embodiment of the present invention are applied. The optical RZmodulator 2 of the clock modulation type shown in FIG. 3 is common tothat of the first embodiment described hereinabove in that two kinds ofMach-Zehnder optical modulators are formed integrally on the substrate1A made of lithiumniobate (LiNbO₃) and Z-axis cut, but is different inconfiguration of the first Mach-Zehnder optical modulator 11-2.

It is to be noted that, in FIG. 3, like reference characters to those ofFIG. 2 denote substantially like elements. Thus, the second Mach-Zehnderoptical modulator 12-1 is configured in a similar manner to that in thefirst embodiment described hereinabove.

The first Mach-Zehnder optical modulator 11-2 includes a first electrode7A-2 and a bias electrode 7C different from those of the firstMach-Zehnder optical modulator 11-1 in the first embodiment, andincludes characteristic elements (refer to reference characters 13-1 and13-2) of the present embodiment similar to those in the Mach-Zehnderoptical modulator 12-1.

It is to be noted that, while the optical waveguides 5 and 6 and theelements denoted by reference characters recesses 13-1 and 13-2hereinafter described are covered, at portions thereof which overlapwith the grounding electrodes 7, with the grounding electrodes 7, inorder to clearly indicate the arrangement configuration of them, alsothe portions of them covered with the grounding electrodes 7 areindicated by solid lines in FIG. 3.

In particular, the first electrode 7A-2 includes a single signalelectrode 7 a and the grounding electrodes 7. The signal electrode 7 ais formed so as to establish electric connection between two connectorcontacts at edge portions of one side of the substrate 1A in itswidthwise direction similarly to the signal electrode 7 b of the secondMach-Zehnder optical modulator 12-1. The signal electrode 7 a is formedfurther such that part thereof extends along and above the portion atwhich the straight arm waveguide 5B-1 of the first optical waveguide 5is formed.

In other words, the first electrode 7A-2 and the second electrode 7B-1are each formed as a single electrode having the single signal electrode7 a or 7 b.

It is to be noted that, since the first electrode 7A-2 and the secondelectrode 7B-1 in the second embodiment include the single signalelectrodes. 7 a and 7 b, respectively, the second embodiment need notinclude a phase delaying section (reference character 9A of FIG. 1) forapplying a clock signal voltage to the dual electrode like the firstelectrode 7A-1 in the first embodiment.

Further, the bias electrode 7C applies a dc voltage from the dc powersupply 7F as a single electrode to the first optical waveguide 5 and isformed such that it extends along and above the straight arm waveguide5B-1. Also the bias electrode 7C described above includes a connectionpad 7 d similarly to the bias electrode 7D of the second Mach-Zehnderoptical modulator 12-1.

Also the optical modulator 11-2 includes a first recess 13-1 and asecond recess 13-2 as characteristic elements of the present inventionfor reducing the loss by the waveguide and suppressing the loss of amicrowave which advances through the signal electrode 7 a similarly asin the optical modulator 12-1. In particular, the sectional structure ofthe optical modulator 11-2 taken along line R-S in FIG. 3 is basicallysimilar to that described hereinabove with reference to FIG. 1 exceptthat the signal electrode 7 a is formed on the ridge 14 a.

In other words, the optical modulator 2 in the second embodiment isformed from two optical modulators 11-2 and 12-1 respectively having therecesses 13-1 and 13-2, which are characteristic elements of the presentinvention, and integrated integrally on one chip.

Also in the optical modulator 2 in the second embodiment having theconfiguration described above, when light from the light source notshown is introduced into it, while the light propagates in the firstoptical waveguide 5 which forms the optical modulator 11-2, it can beconverted into an optical RZ signal of 40 GHz with a sine wave of 20 GHzby the first electrode 7A-2 to which an RZ signal of a frequency of 20GHz generated by the clock generation drive section 8A is applied.

Further, while the optical RZ signal propagates in the second opticalwaveguide 6 which forms the optical modulator 12-1, an NRZ signal of 40Gb/s generated by the NRZ data signal generator 10 is applied to thesecond electrode 7B-1 to modulate the optical clock thereby to modulatethe optical RZ signal into an optical RZ data signal of 40 Gb/s. Theoptical RZ data signal is transmitted to the reception side through anoptical fiber or the like not shown.

Since the second recess 13-2 is provided on the second bank 14 c-2 onwhich the second arm waveguides 5B-2 and 6B-2 of the first opticalwaveguide 5 and the second optical waveguide 6 described above areformed, the structures of the portions at which the second armwaveguides 5B-2 and 6B-2 are formed can be formed substantially same asthe structure of the ridges 14 a which has the signal electrodes 7 a and7 b provided on the tops thereof. Consequently, the losses by the armwaveguides 5B-1, 5B-2. 6B-1 and 6B-2 are substantially equal to eachother.

Further, similarly as in the first embodiment, since the first recess13-1 is provided at the position symmetrical with the second recess 13-2with respect to the ridge 14 a on the first bank 14 c-1 on which the twosets of straight arm waveguides 5B-1, 5B-2 and 6B-1, 6B-2 are notformed, also the electric field distribution in the substrate 1A byelectric signals provided to the signal electrodes 7 a and 7 b can bemade symmetrical with respect to the ridge 14 a.

In this manner, with the optical modulators 11-2 and 12-1 according tothe second embodiment of the present invention, since the first recess13-1 and the second recess 13-2 are formed at symmetrical positions withrespect to the ridge 14 a on the first bank 14 c-1 and the second bank14 c-2, respectively, also the electric field distribution in thesubstrate 1A by an electric signal provided to the signal electrode 7 bcan be made symmetrical with respect to the ridge 14 a similarly as inthe first embodiment described hereinabove. Consequently, the opticalmodulators 11-2 and 12-1 are advantageous in that the loss of amicrowave which advances in the signal electrodes 7 a and 7 b can besuppressed.

Further, since the provision of the second recess 13-2 on the opticalmodulators 11-2 and 12-1 makes it possible to form the structure of theportions at which the second arm waveguides 5b-2 and 6B-2 are formedsubstantially same as the structure of the ridge 14 a which has thesignal electrodes 7 a and 7 b formed on the top thereof, the opticalmodulators 11-2 and 12-1 are advantageous also in that the losses by thearm waveguides 5B-1, 5B-2, 6B-1 and 6B-2 can be made substantially equalto each other thereby to suppress the deterioration of the extinctionratio.

Furthermore, since the buffer layer 1D is formed between the face of thesubstrate 1A and the first electrode 7A-2 and second electrode 7B-1, theoptical modulators 11-2 and 12-1 are further advantageous in that theabsorption losses of light which propagates in the optical waveguides 5and 6 can be suppressed and electric charge generated by a pyroelectriceffect can be made uniform by the silicon layer 1E thereby to suppressthe variation of the operating point by a temperature variation.

Besides, since the buffer layer 1D and the silicon layer 1E are formedalso in the recesses 13-1 and 13-2, adjustment of the characteristicimpedance, which should be kept to a fixed value set in advance, and thevelocity match between a microwave and light can be performed readily bysetting of the thickness of the buffer layer ID and the silicon layer1E.

c. Third Embodiment

FIG. 4 is a schematic view showing an optical RZ modulator of the clockmodulation type to which an optical modulator 11-2 according to a thirdembodiment of the present invention is applied. Also the optical RZmodulator 3 of the clock modulation type shown in FIG. 4 is common tothose of the first and second embodiments described hereinabove in thattwo kinds of Mach-Zehnder optical modulators are formed integrally onthe substrate 1A made of lithium niobate (LiNbO₃) and Z-axis cut, but isdifferent in configuration of a second Mach-Zehnder optical modulator12-2.

It is to be noted that, in FIG. 4, like reference characters to those ofFIG. 3 denote substantially like elements. Thus, the first Mach-Zehnderoptical modulator 11-2 having recesses 13-1 and 13-2 which arecharacteristic elements of the present invention is configured in asimilar manner to that in the second embodiment described hereinabove.

It is to be noted that, similarly as in the first and second embodimentsdescribed above, the optical waveguides 5 and 6 and the elements denotedby reference characters 13-1 and 13-2 which are hereinafter describedare indicated, at portions thereof which are covered with groundingelectrodes 7, by solid lines in FIG. 4.

The second Mach-Zehnder optical modulator 12-2 includes a secondelectrode 7B-2 and bias electrodes 7D-1 and 7D-2 different from those ofthe second Mach-Zehnder optical modulator 12-1 in the first and secondembodiments, but does not include recesses as characteristic elements ofthe present invention.

The second electrode 7B-2 is formed on the substrate 1A for controllinglight which propagates in the second optical waveguide 6 and includes adual electrode having two signal electrodes 7 b-1 and 7 b-2 andgrounding electrodes 7.

The signal electrodes 7 b-1 and 7 b-2 are formed so as to establishelectric connection between two connector contacts at edge portions ofthe opposite sides of the substrate 1A in its widthwise direction. Thesignal electrode 7 b-1 is formed such that part thereof extends alongand above the portion at which the straight arm waveguide 6B-1 of thesecond optical waveguide 6 is formed. The, signal electrode 7 b-2 isformed such that part thereof extends along and above the portion atwhich the other straight arm waveguide 6B-2 of the second opticalwaveguide 6 is formed.

In other words, of the first electrode 7A-2 and the second electrode7B-2 described above, the second electrode 7B-2 is formed as a dualelectrode having two signal electrodes while the first electrode 7A-2 isformed as a single electrode having a single signal electrode.

Further, NRZ data signal generators 10-1 and 10-2 are connected to thesignal electrodes 7 b-1 and 7 b-2 of the second electrode 7B-1 andsupply NRZ data signals of a bit rate corresponding to an optical clockto the second electrode 7B-2.

The NRZ data signal generators 10-1 and 10-2 generate same NRZ datasignals having phases displaced by 180 degrees from each other. The NRZdata signal voltage from the NRZ data generator 10-1 is applied to thesignal electrode 7 b-1, and the NRZ data signal voltage from the NRZdata generator 10-2 is applied to the signal electrode 7 b-2.

It is to be noted that, in FIG. 4, the NRZ data signal generators 10-1and 10-2 are shown such that they generate data signals of 40 Gb/s.

Also in the optical modulator 3 of the third embodiment having theconfiguration described above, when light from the light source notshown is introduced into it, while the light propagates in the firstoptical waveguide 5 which forms the optical modulator 11-2, it can beconverted into an optical RZ signal of 40 GHz with a sine wave of 20 GHzby the first electrode 7A-2 to which an RZ signal of a frequency of 20GHz generated by the clock generation drive section 8A is applied.

Further, while the optical RZ signal propagates in the second opticalwaveguide 6 which forms the optical modulator 12-2, NRZ signals of 40Gb/s generated by the NRZ data signal generators 10-1 and 10-2 areapplied to the signal electrodes 7 b-1 and 7 b-2 of the second electrode7B-1, respectively, to modulate the optical clock thereby to modulatethe optical RZ signal into an optical RZ data signal of 40 Gb/s. Theoptical RZ data signal is transmitted to the reception side through anoptical fiber or the like not shown.

Since the second recess 13-2 is provided on the second bank 14 c-2 onwhich the second arm waveguide 5B-2 of the first optical waveguide 5described above is formed, the structure of the portion at which thesecond arm waveguide 5B-2 is formed can be formed substantially same asthe structure of the ridge 14 a which has the signal electrode 7aprovided on the top portion thereof. Consequently, the losses by thearm waveguides 5B-1 and 5B-2 are substantially equal to each other.

Further, similarly as in the second embodiment since the first recess13-1 is provided at the position symmetrical with the second recess 13-2with respect to the ridge 14 a on the first bank 14 c-1 on which the armwaveguides 5B-1 and 5B-2 are not formed, also the electric fielddistribution in the substrate 1A by electric signals provided to thesignal electrodes 7 a and 7 b can be made symmetrical with respect tothe ridge 14 a.

In this manner, also with the optical modulator 3 according to the thirdembodiment of the present invention, since the optical modulator 11-2 isformed integrally and the first recess 13-1 and the second recess 13-2are formed at symmetrical positions with respect to the ridge 14 a onthe first bank 14 c-1 and the second bank 14 c-2, respectively, similaradvantages to those of the first embodiment described hereinabove can beachieved.

Further, since the buffer layer 1D is formed between the face of thesubstrate 1A and the first electrode 7A-2 and second electrode 7B-2, theoptical modulator 3 is further advantageous in that the absorptionlosses of light which propagates in the optical waveguides 5 and 6 canbe suppressed and electric charge generated by a pyroelectric effect canbe made uniform by the silicon layer 1E thereby to suppress thevariation of the operating point by a temperature variation.

Besides, since the buffer layer 1D and the silicon layer 1E are formedalso in the recesses 13-1 and 13-2, adjustment of the characteristicimpedance, which should be kept to a fixed value set in advance, and thevelocity match between a microwave and light can be performed readily bysetting of the thickness of the buffer layer 1D and the silicon layer1E.

d. Fourth Embodiment

In the first to third embodiments described in detail above, an opticalmodulator having the recesses 13-1 and 13-2 which are characteristicelements of the present invention is applied to an optical RZ modulatorof the clock modulation type. However, according to the presentinvention, it is naturally possible, for example, to form an opticalmodulator as a single drive optical modulator 12 of a single unit havinga basically similar configuration to that of the optical modulators 11-2and 12-1 of the first to third embodiments described hereinabove, asshown in FIG. 5

In particular, the optical modulator 12 according to the fourthembodiment can be used as a single drive modulator which performs NRZdata modulation of light from a light source similarly to the opticalmodulator 20 described hereinabove with reference to FIG. 8. However,the optical modulator 12 is different from the optical modulator 20 inthat it has recesses 13-1 and 13-2 which are characteristic elements ofthe present invention.

It is to be noted that, similarly as in the first and second embodimentsdescribed above, the optical waveguide 6 and the elements denoted byreference characters 13-1 and 13-2 are indicated, at portions thereofwhich are covered with a grounding electrode 7, by solid lines in FIG.5.

Also in the optical modulator 12 according to the fourth embodiment,similarly as in the optical modulators 11-2 and 12-1 in the embodimentsdescribed hereinabove, the sectional structure taken along line R-S ofFIG. 5 is configured basically similarly to that described hereinabovewith reference to FIG. 1. Consequently, the optical modulator 12 canreduce the loss of the waveguide and suppress the loss of a microwavewhich advances in the signal electrode 7 b. It is to be noted that, inFIG. 4, like reference characters similar to those of FIG. 1 denotesubstantially like elements.

In the single drive optical modulator 12 having the configurationdescribed above with reference to FIG. 4, when light from the lightsource not shown is introduced into it, while the light propagates inthe Mach-Zehnder optical waveguide, it is modulated into an NRZ opticalsignal by the signal electrode 7 b to which a voltage signal of NRZ dataor the like is applied. The NRZ optical signal is outputted from thesingle drive optical modulator 12.

Since the second recess 13-2 is provided on the second bank 14 c-2 onwhich the second arm waveguide 6B-2 of the optical waveguide 6 describedabove is formed, the structure of the portion at which the second armwaveguide 6B-2 is formed can be formed substantially same as thestructure of the ridge 14 a which has the signal electrode 7 b providedat the top portion thereof. Consequently, the losses by the straight armwaveguides 6B-1 and 6B-2 are substantially equal to each other.

Further, since the first recess 13-1 is provided at the positionsymmetrical with the second recess 13-2 with respect to the ridge 14 aon the first bank 14 c-1 on which the two arm waveguides 6B-1 and 6B-2are not formed, also the electric field distribution in the substrate 1Aby an electric signal provided to the signal electrode 7 b can be madesymmetrical with respect to the ridge 14 a.

In this manner, also with the optical modulator according to the fourthembodiment, similar advantages to those of the first to thirdembodiments described hereinabove can be achieved.

e. Others

It is to be noted that, while, in the optical modulators of theembodiments described hereinabove, an air gap is formed in each of therecesses 13-1 and 13-2, that is, in a region defined by each of therecesses 13-1 and 13-2 and the grounding electrode 7, the opticalmodulator according to the present invention is not limited to thespecific configuration and may otherwise be configured such that, forexample, as shown in FIG. 6, a grounding electrode 7 is provided aboveeach of the first recess 13-1 and the second recess 13-2 and part of thegrounding electrode 7 is filled in each of the first recess 13-1 and thesecond recess 13-2.

Further, while, in the embodiments described above, the buffer layer 1Dis formed between the substrate 1A and the first electrode 7A-1 andsecond electrode 7B-1 and the silicon layer 1E is formed on the bufferlayer 1D over the overall area of the substrate 1A (refer to FIG. 1),according to the present invention, it is otherwise possible toconfigure the optical modulator such that, for example, as shown in across sectional view of FIG. 7 taken along line R-S, a buffer layer 1D′and a silicon layer 1E are formed between at least the portions of thesubstrate 1A on which the optical waveguides 5 and 6 are formed and thefirst electrode 7A-1 and second electrode 7B-1. Also where the opticalmodulator is configured in this manner, at least the absorption lossesof light which propagates in the optical waveguides 5 and 6 can besuppressed.

Further, while the modulators of the embodiments described above inwhich the recesses 13-1 and 13-2 are provided are shown such that thewidth of the first arm waveguides 5B-1 and 6B-1 is substantially equalto the contact width of the signal electrode 7 b with the ridge 14 a,according to the present invention, the relationship of the width Rw ofthe ridge 14 a, the waveguide width and the contact width We between thesignal electrode 7 b and the ridge 14 a can be varied within a rangewithin which the balance among the demand for suppression of thewaveguide loss, the demand for lowering of the drive voltage and thedemand for the velocity match between microwave and light can bemaintained taking the electrode distance Se and the electrode thicknessinto consideration.

Furthermore, while the recesses 13-1 and 13-2 are each formed in a shapeof a continuous groove such that they extend in parallel to the firstarm waveguides 5B-1 and 6B-1 and the second arm waveguides 5B-2 and 6B-2and have a length substantially equal to the length L of the groundingelectrode 7 on the upper faces of the second arm waveguide 5B-2 and6B-2, similar advantages to those which can be achieved by theembodiments described hereinabove can be achieved even where the opticalmodulator is formed such that the length of the continuous groove shapeof the recesses 13-1 and 13-2 is varied or each of the recesses 13-1 and13-2 has a shape of an intermittent groove.

Further, while the optical modulates of the embodiments described abovecan modulate a data signal into an optical signal having an informationamount of 40 Gb/s, the present invention can be applied to anotheroptical modulator for modulation into an optical signal having aninformation amount smaller than 40 Gb/s or a further optical modulatorfor modulation into an optical signal having an information amountgreater than 40 Gb/s.

The present invention is not limited to the embodiments specificallydescribed above, and variations and modifications can be made withoutdeparting from the scope of the present invention.

What is claimed is:
 1. An optical modulator, comprising: a substratehaving an electro-optical effect and having formed thereon a ridge,first and second grooves which are positioned on the opposite sides ofthe ridge, and first and second banks which are positioned on the outersides of the first and second grooves, respectively; a first opticalwaveguide formed in the ridge on the substrate; a second opticalwaveguide formed on the substrate; an electrode formed on the substrateand including a signal electrode formed on the ridge and two groundingelectrodes formed on the first and second banks for controlling lightpropagating in the first and second optical waveguides; and first andsecond recesses formed at symmetrical positions with respect to theridge on the first and second banks, respectively.
 2. An opticalmodulator as claimed in claim 1, wherein the substrate is made ofLiNbO₃.
 3. An optical modulator as claimed in claim 2, wherein thesubstrate made of LiNbO₃ is a Z-axis cut substrate.
 4. An opticalmodulator as claimed in claim 1, wherein the ridge and the first andsecond banks have top faces which are set in a substantially same levelwith one another.
 5. An optical modulator as claimed in claim 4, whereinthe first and second recesses have a depth set substantially equal tothe depth of the first and second grooves.
 6. An optical modulator asclaimed in claim 1, wherein a buffer layer is formed between thesubstrate and the electrode.
 7. An optical modulator as claimed in claim6, wherein the buffer layer is provided also in the first and secondrecesses.
 8. An optical modulator as claimed in claim 1, wherein asilicon layer is placed on the substrate.
 9. An optical modulator asclaimed in claim 8, wherein the silicon layer is provided also in thefirst and second recesses.
 10. An optical modulator as claimed in claim1, wherein the grounding electrodes are provided on the first and secondrecesses and an air gap is formed in each of the first and secondrecesses.
 11. An optical modulator as claimed in claim 1, wherein thegrounding electrodes are provided on the first and second recesses andpart of the grounding electrodes is filled in the first and secondrecesses.
 12. An optical modulator as claimed in claim 1, wherein thesignal electrode contacts with the ridge with a width smaller than thewidth of the ridge.
 13. An optical modulator as claimed in claim 1,wherein the second optical waveguide is provided at a location of one ofthe first and second banks nearer to the ridge than a corresponding oneof the first and second recesses.
 14. An optical modulator, comprising:a Z-axis cut substrate made of LiNbO₃ and having formed thereon a ridge,first and second grooves which are positioned on the opposite sides ofthe ridge, and first and second banks which are positioned on the outersides of the first and second grooves, respectively; a first opticalwaveguide formed in the ridge on the substrate; a second opticalwaveguide formed on the substrate; an electrode formed on the substrateand including a signal electrode formed on the ridge and two groundingelectrodes formed on the first and second banks, for controlling lightpropagating in the first and second optical waveguides; a buffer layerformed between the substrate and the electrode; a silicon layer placedon the substrate; and first and second recesses formed at symmetricalpositions with respect to the ridge on the first and second banks,respectively.
 15. An optical modulator as claimed in claim 14, whereinthe buffer layer is provided also in the first and second recesses. 16.An optical modulator as claimed in claim 14, wherein the silicon layeris provided also in the first and second recesses.
 17. An opticalmodulator as claimed in claim 14, wherein the second optical waveguideis provided at a location of one of the first and second banks nearer tothe ridge than a corresponding one of the first and second recesses. 18.An optical modulator comprising: a ridge with first and second groovespositioned on opposite sides of the ridge; first and second bankspositioned on outer sides of the first and second grooves, respectively;first and second recesses in the first and second banks, respectively,the first and second recesses formed at symmetrical positions withrespect to the ridge; and first and second optical waveguides, one ofthe first and second optical waveguides, but not the other of the firstand second optical waveguides, being on the ridge.
 19. An opticalmodulator as in claim 18, further comprising: a substrate having anelectro-optical effect, wherein the ridge, the first and second grooves,the first and second banks, the first and second recesses and the firstand second optical waveguides are formed on the substrate.
 20. Anoptical modulator as in claim 19, wherein the substrate is made ofLiNbO₃.
 21. An optical modulator as in claim 19, wherein the substrateis a z-cut substrate made of LiNbO₃.
 22. An optical modulator as inclaim 19, further comprising first and second grounding electrodes onthe first and second banks, respectively.
 23. An optical modulator as inclaim 19, further comprising first and second grounding electrodes onthe first and second banks, respectively, wherein at least part of thefirst and second grounding electrodes are filled in the first and secondrecesses, respectively.
 24. An optical modulator as in claim 19, whereinthe ridge and the first and second banks have top faces which are set ina substantially same level with each other.
 25. An optical modulator asin claim 19, wherein the first and second recesses and the first andsecond grooves each have a substantially same depth.
 26. An opticalmodulator as in claim 19, further comprising a signal electrode on theridge, the signal electrode contacting the ridge with a width smallerthan a width of the ridge.
 27. An optical modulator as in claim 19,further comprising: a signal electrode on the ridge; first and secondgrounding electrodes on the first and second banks, respectively; and abuffer layer between the substrate and each of the first groundingelectrode, the second grounding electrode and the signal electrode. 28.An optical modulator as in claim 18, wherein said other of the first andsecond optical waveguides is provided on one of the first and secondbanks and at a position nearer to the ridge than the respective recessof the first and second recesses which is in said one of the first andsecond banks.
 29. An optical modulator comprising: a ridge with firstand second grooves positioned on opposite sides of the ridge; first andsecond banks positioned on outer sides of the first and second grooves,respectively; first and second recesses in the first and second banks,respectively, the first and second recesses formed at symmetricalpositions with respect to the ridge; first and second opticalwaveguides, one of the first and second optical waveguides, but not theother of the first and second optical waveguides, being on the ridge;and a z-cut LiNbO₃ substrate having an electro-optical effect, whereinthe ridge, the first and second grooves, the first and second banks, thefirst and second recesses and the first and second optical waveguidesare formed on the substrate.
 30. An optical modulator as in claim 29,further comprising first and second grounding electrodes on the firstand second banks, respectively.
 31. An optical modulator as in claim 29,further comprising first and second grounding electrodes on the firstand second banks, respectively, wherein at least part of the first andsecond grounding electrodes are filled in the first and second recesses,respectively.
 32. An optical modulator as in claim 29, wherein the ridgeand the first and second banks have top faces which are set in asubstantially same level with each other.
 33. An optical modulator as inclaim 29, wherein the first and second recesses and the first and secondgrooves each have a substantially same depth.
 34. An optical modulatoras in claim 29, further comprising a signal electrode on the ridge, thesignal electrode contacting the ridge with a width smaller than a widthof the ridge.
 35. An optical modulator as in claim 29, furthercomprising: a signal electrode on the ridge; first and second groundingelectrodes on the first and second banks, respectively; and a bufferlayer between the substrate and each of the first grounding electrode,the second grounding electrode and the signal electrode.
 36. An opticalmodulator as in claim 29, wherein said other of the first and secondoptical waveguides is provided on one of the first and second banks andat a position nearer to the ridge than the respective recess of thefirst and second recesses which is in said one of the first and secondbanks.
 37. An optical modulator comprising: a ridge with first andsecond grooves positioned on opposite sides of the ridge; first andsecond banks positioned on outer sides of the first and second grooves,respectively; first and second recesses in the first and second banks,respectively, the first and second recesses formed at symmetricalpositions with respect to the ridge; first and second opticalwaveguides, one of the first and second optical waveguides, but not theother of the first and second optical waveguides, being on the ridge; asignal electrode on the ridge; first and second grounding electrodes onthe first and second banks, respectively; and a z-cut LiNbO₃ substratehaving an electro-optical effect, wherein the ridge, the first andsecond grooves, the first and second banks, the first and secondrecesses, the first and second optical waveguides, the signal electrode,and the first and second grounding electrodes are formed on thesubstrate.
 38. An optical modulator as in claim 37, wherein at leastpart of the first and second grounding electrodes are filled in thefirst and second recesses, respectively.
 39. An optical modulator as inclaim 37, wherein the ridge and the first and second banks have topfaces which are set in a substantially same level with each other. 40.An optical modulator as in claim 37, wherein the first and secondrecesses and the first and second grooves each have a substantially samedepth.
 41. An optical modulator as in claim 37, wherein the signalelectrode contacts the ridge with a width smaller than a width of theridge.
 42. An optical modulator as in claim 37, further comprising: abuffer layer formed on the substrate, the buffer layer being between thesubstrate and each of the first grounding electrode, the secondgrounding electrode and the signal electrode.
 43. An optical modulatoras in claim 37, wherein said other of the first and second opticalwaveguides is provided on one of the first and second banks and at aposition nearer to the ridge than the respective recess of the first andsecond recesses which is in said one of the first and second banks.